This invention relates to the preparation of novel strains of yeast and to the use thereof for simultaneously preparing by fermentation a biological material which is enriched in L-lysine and which may be used as food, and citric acid.
Yeasts of the species Candida lipolytica are known and have been grown on hydrocarbon-containing substrates.
It has now been found that yeasts of the species Candida lipolytica can be mutated to produce new strains which are resistant to lysine analogues which, in low concentrations, are normally toxic to these yeasts.
When the strains acquire the aforementioned resistance, there is a considerable increase in the lysine intracellular ratio and they can simultaneously excrete appreciable quantities of citric and isocitric acids in a fermentation medium containing hydrocarbons and mineral salts in non-limiting quantities, without it being necessary to add any compound capable of producing one or other of these acids during fermentation.